Fuel cell system

ABSTRACT

A fuel cell system determines whether an operation condition of a fuel cell corresponds to a fuel gas shortage or an oxidation gas shortage. Upon determining that the fuel gas is in shortage, the system sets a lower voltage limit to be higher than that set when the system determines that the oxidation gas is in shortage. The system further controls an output voltage from the fuel cell so as to prevent output voltage from the fuel cell from decreasing below the lower voltage limit.

TECHNICAL FIELD

The present invention relates to a fuel cell system including a fuelcell that receives a supply of a fuel gas and an oxidation gas togenerate power.

BACKGROUND ART

A fuel cell stack has a stack structure with a plurality of cellsstacked in series. Each of the cells includes a membrane-electrodeassembly with an anode located on one surface of an electrolyticmembrane and a cathode located on the other surface thereof. Supplying afuel gas and an oxidation gas to the membrane-electrode assembly inducesan elecrtrochemical reaction. Thus, chemical energy is converted intoelectric energy. In particular, a solid polyelectrolyte fuel stack usinga solid polymer film as an electrolyte requires reduced costs, is easyto make compact, and offers a high power density. The solidpolyelectrolyte fuel stack is thus expected to be used as avehicle-mounted power source.

To operate a fuel cell system, moisture inside the fuel cell stack andthe temperature of the interior thereof need to be closely managed todetect a power generation error such as a shortage of a supply of areaction gas caused by flooding or an increase in resistance caused bythe dried electrolytic membrane. Thus, the power generation condition ofthe cells needs to be managed so as to provide sufficient power.Japanese Patent Publication No. 7-63020 refers to a scheme ofcontrolling operation so as to prevent an electrode potential fromincreasing above an upper voltage limit while preventing a cell voltagefrom decreasing below a lower voltage limit (the electrode potentialrefers to the potential of each of the anode and the cathode, and thecell voltage refers to the potential of the cathode with reference tothe anode). When the fuel cell stack continues to be operated at a highpotential at which the electrode potential is in an oxidation region, acatalyst may be ionized and eluted. Thus, the upper voltage limit isspecified so as to prevent the electrode potential from increasingenough to degrade the catalyst. Furthermore, when power generation iscontinued with some of the cells offering a reverse potential (in thisphenomenon, the anode potential is higher than the cathode potential),the catalyst degradation or the like may occur to deteriorate powergeneration performance. Thus, the lower voltage limit is specified so asto prevent the possible reverse potential.

[Patent Document 1] Japanese Patent Publication No. 7-63020

DISCLOSURE OF THE INVENTION

However, with the conventional operation scheme, since the lower voltagelimit has a predetermined fixed value, a tolerable range of powergeneration cannot be flexibly changed depending on a cell voltagereduction factor. Disadvantageously, the tolerable range of powergeneration is fixed.

Furthermore, if a fuel cell stack made up of a plurality of cellsstacked in series is operated, completely equalizing the powergeneration performance and conditions of all the cells is difficult.When operation conditions are degraded owing to a decrease in protonconductivity caused by a dry-up phenomenon, the shortage of the supplyof the reaction gas, or the like, the cell voltage decreases in cellswith low power generation performance or in improper conditions. Amongthe cell voltage reduction factors, a shortage of the fuel gas mayelectrochemically damage the fuel cell stack. When the cell voltagereduction factor is a shortage of the oxidation gas, the fuel cell stackis prevented from being electrochemically damaged.

However, with the conventional scheme, determining whether the cellvoltage reduction factor is the fuel gas shortage or the oxidation gasshortage is difficult. Thus, when a decrease in cell voltage isdetected, output is equally limited even if the cell voltage reductionfactor is the oxidation gas shortage. As a result, drivability may bedegraded.

Thus, an object of the present invention is to provide a fuel cellsystem which can solve the above-described problems and flexibly changethe tolerable range of power generation depending on the operationconditions. Another object of the present invention is to provide a fuelcell system that can determine whether the cell voltage reduction factoris the fuel gas shortage or the oxidation gas shortage to properlymanage the cell voltage according to the cell voltage reduction factor.

To accomplish these objects, a fuel cell system according to the presentinvention comprises a fuel cell which receives a supply of a fuel gasand an oxidation gas to generate power, a determination device whichdetermines whether or not an operation condition of the fuel cellcorresponds to a fuel gas shortage or an oxidation gas shortage, a lowervoltage limit setting device which, when the determination devicedetermines that the fuel gas is in shortage, sets a lower voltage limitto be higher than that set when the determination device determines thatthe oxidation gas is in shortage, and a control device which controls anoutput voltage from the fuel cell so as to prevent the output voltagefrom decreasing below the lower voltage limit set by the lower voltagelimit setting device.

If the cell voltage reduction factor is the fuel gas shortage,continuous power generation with the cell offering a reverse potentialmay severely damage the cell. Thus, the lower voltage limit ispreferably set to be higher to strictly limit the decrease in cellvoltage so as to prevent the possible reverse potential. On the otherhand, if the cell voltage reduction factor is the oxidation gasshortage, the continuous power generation with the cell offering thereverse potential less severely damages the cell than when the fuel gasis in shortage. Thus, to allow the reverse potential to be generated,the lower voltage limit for the oxidation gas shortage is preferably setto be lower than that for the fuel gas shortage to relatively ease thelimitation on the decrease in cell voltage.

A fuel cell system based on another aspect of the present inventioncomprises a fuel cell which receives a supply of a fuel gas and anoxidation gas to generate power, a lower voltage limit setting devicewhich sets a lower voltage limit for a low-efficiency operation to belower than that for a normal operation, and a control device thatcontrols an output voltage from the fuel cell so as to prevent theoutput voltage from decreasing below the lower voltage limit set by thelower voltage limit setting device.

A decrease in the temperature of the fuel cell reduces the activity ofan electrochemical reaction that may damage the fuel cell. Thus, thelower voltage limit is preferably set to a value decreasing consistentlywith the temperature of the fuel cell to ease the limitation on thedecrease in cell voltage. For example, the lower voltage limit for thelow-efficiency operation is preferably set to be lower than that for thenormal operation.

In this case, the control device preferably performs the low-efficiencyoperation when temperature is equal to or lower than a predeterminedvalue at the time of the starting of the fuel cell system. Performingthe low-efficiency operation controls a heat loss in the fuel cell toallow the fuel cell to warm up early.

The lower voltage limit setting device preferably sets the lower voltagelimit to be lower when the fuel gas is in shortage during thelow-efficiency operation than when the fuel gas is in shortage duringthe normal operation.

When the output voltage from the fuel cell is lower than the lowervoltage limit, the control device desirably attempts to recover the cellvoltage by carrying out one of a process of increasing an amount of fuelgas or oxidation gas supplied to the fuel cell, a process of limitingthe output current from the fuel cell, and a process of stopping powergeneration.

In a transitive power generation condition with a variation in load,when a variation in the output voltage from the fuel cell is defined asΔV and a variation in output current from the fuel cell is defined asΔI, the determination device calculates |ΔV/ΔI|, and when |ΔV/ΔI| isequal to or larger than a first threshold, determines that the fuel gasis in shortage. When |ΔV/ΔI| is smaller than the first threshold, thedetermination device determines that the oxidation gas is in shortage.

When the fuel gas is in shortage, an insufficient amount of protons iscompensated for by electrolysis of water. Thus, |ΔV/ΔI| is larger thanthe first threshold. When the oxidation gas is in shortage, the ohmicresistance of an electrolytic membrane becomes dominant to reduce|ΔV/ΔI| below the first threshold. Comparison of |ΔV/ΔI| with the firstthreshold enables the fuel gas shortage to be distinguished from theoxidation gas shortage.

In a steady-state power generation condition with no variation in load,when a variation in the output voltage from the fuel cell is defined asΔV and a temporal variation is defined as Δt, the determination devicecalculates |ΔV/Δt|, and when |ΔV/Δt| is equal to or larger than a secondthreshold, determines that the fuel gas is in shortage. When |ΔV/Δt| issmaller than the second threshold, the determination device determinesthat the oxidation gas is in shortage.

When the fuel gas is in shortage, an insufficient amount of protons iscompensated for by electrolysis of water. Thus, |ΔV/Δt| is larger thanthe second threshold. When the oxidation gas is in shortage, the ohmicresistance of the electrolytic membrane becomes dominant to reduce|ΔV/Δt| below the second threshold. Comparison of |ΔV/Δt| with thesecond threshold enables the fuel gas shortage to be distinguished fromthe oxidation gas shortage.

In a preferred embodiment of the present invention, the determinationdevice actually measures an anode potential of the fuel cell. When theanode potential of the fuel cell is higher than a predeterminedthreshold potential, the determination device determines that the fuelgas is in shortage. When the anode potential of the fuel cell is lowerthan the predetermined threshold potential, the determination devicedetermines that the oxidation gas is in shortage. When the fuel gas isin shortage, the fuel cell applies electrolysis to water to generateprotons to compensate for the fuel gas shortage. At this time, since theanode potential is higher than a certain threshold potential, comparisonof the anode potential with the threshold potential allows determinationof whether the fuel gas or the oxidation gas is in shortage. When theoxidation gas is determined to be in shortage, since the cell voltage isallowed to exhibit a negative value, the need for a limitation on anoutput from the fuel cell stack is eliminated. Thus, possibledegradation of drivability can be inhibited.

The determination device calculates the cell voltage on acurrent-voltage characteristic map created under a condition whichminimizes an increase in the anode potential of the fuel cell during theelectrolysis of water. When the actual cell voltage is higher than thecell voltage on the current-voltage characteristic map, thedetermination device determines that the fuel gas shortage is notoccurring. The condition which minimizes the increase in the anodepotential of the fuel cell during the electrolysis of water refers to,for example, the condition under which a sufficient amount of moisturerequired to generate protons is present inside the fuel cell. Even ifthe cell voltage drops to a negative value owing to any factor, the cellvoltage reduction factor is ensured not at least to be the fuel gasshortage as long as the current cell voltage belongs to an upper regionof the current-voltage characteristic map described above. If the cellvoltage reduction factor is not at least the fuel gas shortage, sincethe cell voltage is allowed to exhibit a negative value, the need forthe limitation on the output from the fuel cell stack is eliminated.Thus, the possible degradation of the drivability can be inhibited.

Here, the current-voltage characteristic map may be (1) map data withtemperature characteristics, (2) map data pre-corrected taking intoaccount control delay time required to control the cell voltage, or (3)map data exhibiting a constant voltage value regardless of cell current.Using the map data for which the temperature characteristics or thecontrol delay time is taken into account enables the cell voltage to bemore closely controlled. On the other hand, using the map dataexhibiting the constant voltage value regardless of the cell currentenables the control of the cell voltage to be simplified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of a fuel cell system 10according to the present embodiment;

FIG. 2 is an exploded perspective view of a cell making up a fuel cellstack;

FIG. 3 is a flowchart showing a cell voltage management routine during anormal operation;

FIG. 4 is a graph showing an I-V characteristic of the fuel cell stackobserved when a reaction gas is in shortage in a transitive powergeneration condition with a variation in load;

FIG. 5 is a graph showing a V-t characteristic of the fuel cell stackobserved when the reaction gas is in shortage in a steady-state powergeneration condition with no variation in load;

FIG. 6 is a flowchart showing a low-temperature starting routine;

FIG. 7 is a diagram illustrating cell voltage management based on a cellelectrode potential;

FIG. 8 is a diagram illustrating cell voltage management based on theI-V characteristic;

FIG. 9 is a diagram illustrating the cell voltage management based onthe I-V characteristic;

FIG. 10 is a diagram illustrating the cell voltage management based onthe I-V characteristic; and

FIG. 11 is a diagram illustrating the cell voltage management based onthe I-V characteristic.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment according to the present invention will be described withreference to the drawings.

FIG. 1 shows a configuration of a fuel cell system 10 according to thepresent embodiment.

The fuel cell system 10 functions as a vehicle-mounted power supplysystem mounted in a fuel cell vehicle. The fuel cell system 10 includesa fuel cell stack 20 that receives a supplied reaction gas (fuel gas andoxidation gas) to generate power, an oxidation gas supply line 30through which air as an oxidation gas is supplied to the fuel cell stack20, a fuel gas supply line 40 through which a hydrogen gas as a fuel gasis supplied to the fuel cell stack 20, a power line 50 that controlscharging and discharging of power, a cooling system 60 through which thefuel cell stack 20 is cooled, and a controller (ECU) 70 that integrallycontrols the whole fuel cell system 10.

The fuel cell stack 20 is a solid polyelectrolyte cell stack made up ofa plurality of cells stacked in series. In the fuel cell stack 20, anoxidation reaction expressed by Formula (1) occurs in an anode, and areduction reaction expressed by Formula (2) occurs in a cathode. For thewhole fuel cell stack 20, an electromotive reaction expressed by Formula(3) occurs.

H₂→2H⁺+2e⁻  (1)

(1/2)O₂+2H⁺+2e⁻→H₂O   (2)

H₂₊(1/2)O₂→H₂O   (3)

The fuel cell stack 20 includes a voltage sensor 71 attached thereto todetect an output voltage from the fuel cell stack 20, a current sensor72 attached thereto to detect a generated current, and a cell monitor(cell voltage detector) 73 attached thereto to detect the voltage ofeach of the cells. The cell monitor 73 may detect the cell voltage foreach cell or for every plural cells (cell module).

The oxidation gas supply line 30 includes an oxidation gas passage 34through which an oxidation gas to be supplied to the cathode of the fuelcell stack 20 flows, and an oxidation off gas passage 36 through whichan oxidation off gas discharged from the fuel cell stack 20 flows. Theoxidation gas passage 34 includes an air compressor 32 that takes in theoxidation gas from atmosphere via a filter 31, a humidifier 33 thathumidifies the oxidation gas to be supplied to the cathode of the fuelcell stack 20, and a throttle valve 35 that adjusts the amount ofoxidation gas supplied. The oxidation off gas passage 36 includes abackpressure regulating valve 37 that regulates an oxidation gas supplypressure, and a humidifier 33 that exchanges moisture between theoxidation gas (dry gas) and the oxidation off gas (wet gas).

The fuel gas supply line 40 includes a fuel gas supply source 41, a fuelgas passage 45 through which the fuel gas to be fed from the fuel gassupply source 41 to the anode of the fuel cell stack 20 flows, acirculation passage 46 through which a fuel off gas discharged by thefuel cell stack 20 is returned to the fuel gas passage 45, a circulationpump 47 through which the fuel off gas in the circulation passage 46 isfed to the fuel gas passage 43 under pressure, and an exhaust and drainpassage 48 divergently connected to circulation passage 47.

The fuel gas supply source 41 is composed of, for example, a highpressure hydrogen tank or hydrogen-occluded alloy and stores a hydrogengas under a high pressure (for example, 35 MPa to 70 MPa). Opening ashutoff valve 42 allows the fuel gas to flow from the fuel gas supplysource 41 to the fuel gas passage 45. The fuel gas has the pressurethereof reduced to, for example, about 200 kPa by a regulator 43 or aninjector 44 before being supplied to the fuel cell stack 20.

The fuel gas supply source 41 may be composed of a reformer thatgenerates a hydrogen-rich reformed gas from a hydrogencarbide-containing fuel, and a high-pressure gas tank in which thereformed gas generated by the reformer is pressurized and accumulated.

The regulator 43 is a device that regulates an upstream pressure(primary pressure) to a preset secondary pressure, and is composed of,for example, a mechanical pressure reducing valve that reduces theprimary pressure. The mechanical pressure reducing valve includes ahousing in which a backpressure chamber and a pressure regulatingchamber are formed opposite each other across a diaphragm. In thepressure reducing valve, a backpressure in the backpressure chamberreduces the primary pressure to a predetermined pressure in the pressureregulating chamber. The secondary pressure is thus obtained.

The injector 44 is an electromagnetically driven on-off valve thatenables a gas flow rate or gas pressure to be regulated by directlydriving the valve disc at a predetermined driving period by means of anelectromagnetic driving force to separate the valve disc from a valveseat. The injector 44 includes the valve seat with an injection holethrough which a gas fuel such as a fuel gas is injected, a nozzle bodythrough which the gas fuel is fed and guided to the injection hole, andthe valve disc accommodated and held in the nozzle body so as to bemovable in an axial direction (gas flow direction) of the nozzle body toopen and close the injection hole.

An exhaust and drain valve 49 is disposed in the exhaust and drainpassage 48. In response to an instruction from the controller 70, theexhaust and drain valve 49 is actuated to discharge moisture and thefuel off gas containing impurities in the circulation passage 46, to theexterior of the system. Opening the exhaust and drain valve 49 reducesthe concentration of the impurities in the fuel off gas in thecirculation passage 46. Thus, the concentration of hydrogen in the fueloff gas circulating through the circulation line can be increased.

The fuel off gas discharged via the exhaust and drain valve 49 is mixedwith the oxidation off gas flowing through the oxidation off gas passage34 and diluted by a diluter (not shown in the drawings). The circulationpump 47 cyclically supplies the fuel off gas in the circulation line tothe fuel cell stack 20 by means of driving of a motor.

The power line 50 includes a DC/DC converter 51, a battery 52, atraction inverter 53, a traction motor 54, and auxiliary devices 55. TheDC/DC converter 51 includes a function of increasing a DC voltagesupplied by the battery 52 and outputting the increased DC voltage tothe traction inverter 53, and a function of reducing DC power generatedby the fuel cell stack 20 or regenerative power recovered by thetraction motor 54 by means of regenerative braking to charge the battery52. The functions of the DC/DC converter 51 control charging anddischarging of the battery 52. Voltage conversion control performed bythe DC/DC converter 51 controls operation points of (output voltage andoutput current from) the fuel cell stack 20.

The battery 52 functions as a storage source for surplus power, astorage source for regenerative energy during regenerative braking, andan energy buffer for a load variation associated with acceleration anddeceleration of the fuel cell vehicle. For example, a secondary batterysuch as a nickel-cadmium battery, a nickel-hydrogen battery, or alithium secondary battery is suitable as the battery 52.

The traction inverter 53 is, for example, a PWM inverter drivenaccording to a pulse width modulation scheme. In accordance with acontrol instruction from the controller 70, the traction inverter 53converts the DC voltage from the fuel cell stack 20 or the battery 52into a three-phase AC voltage to control the rotating torque of thetraction motor 54. The traction motor 54 is, for example, a three-phaseAC motor making up a power source for the fuel cell vehicle.

The auxiliary devices 55 collectively refer to motors (power sourcesfor, for example, pumps) arranged in the respective sections in the fuelcell system 10, inverters allowing the motors to be driven, and variousvehicle-mounted auxiliary devices (for example, an air compressor, aninjector, a cooling water circulation pump, and a radiator).

A cooling system 60 includes refrigerant passages 61, 62, 63, and 64through which a refrigerant circulating through the interior of the fuelcell stack 20 flows, a circulation pump 65 that feeds the refrigerantunder pressure, a radiator 66 that exchanges heat between therefrigerant and outside air, a three-way valve 67 that switches acirculation path for the refrigerant, and a temperature sensor 74 thatdetects the temperature of the refrigerant. During a normal operationfollowing completion of a warming-up operation, the three-way valve 67is controllably opened and closed such that the refrigerant flowing outfrom the fuel cell stack 20 flows through the refrigerant passages 61and 64 and is cooled in the radiator 66 and then flows through therefrigerant passage 63 into the fuel cell stack 20 again. On the otherhand, during the warming-up operation immediately after the starting ofthe system, the three-way valve 67 is controllably opened and closedsuch that the refrigerant flowing out from the fuel cell stack 20 flowsthough the refrigerant passages 61, 62, and 63 into the fuel cell stack20 again.

The refrigerant temperature represents the temperature (catalysttemperature) of the fuel cell stack 20 and is utilized as an index usedto optimize the operation control of the cell.

The controller 70 is a computer system including a CPU, a ROM, a RAM,and an I/O interface. The controller functions as a control device thatcontrols the relevant sections (the oxidation gas supply line 30, thefuel gas supply line 40, the power line 50, and the cooling system 60)of the fuel cell system 10. For example, upon receiving a startingsignal IG output by an ignition switch, the controller 70 startsoperating the fuel cell system 10. The controller 70 then determines thepower requirement for the whole system based on an accelerator openingdegree signal ACC output by an accelerator sensor and a vehicle speedsignal VC output by a vehicle speed sensor.

The power requirement for the whole system is the sum of vehicletraveling power and auxiliary device power. The auxiliary device powerincludes power consumed by vehicle-mounted auxiliary devices (thehumidifier, the air compressor, a hydrogen pump, the cooling watercirculation pump, and the like), power consumed by devices required todrive the vehicle (a speed change gear, a wheel control device, asteering device, a suspension system, and the like), and power consumedby devices disposed in a passenger space (an air conditioning device, alighting system, an audio system, and the like).

The controller 70 determines the allocation of output power from thefuel cell stack 20 and output power from the battery 52 to control theoxidation gas supply line 30 and the fuel gas supply line 40 so that theamount of power generated by the fuel cell stack 20 equals to targetpower. The controller 70 also controls the DC/DC converter 51 toregulate the output voltage from the fuel cell stack 20 to control theoperation points of (output voltage and output current from) the fuelcell stack 20. Moreover, the controller 70 outputs, for example, ACvoltage instruction values for a U phase, a V phase, and a W phase tothe traction inverter 53 as switching instructions to control the outputtorque and rotation speed of the traction motor 54 so as to obtain thetarget vehicle speed corresponding to the accelerator opening degree.

FIG. 2 is an exploded perspective view of cells 21 making up the fuelcell stack 20.

Each of the cells 21 is composed of an electrolytic membrane 22, ananode 23, a cathode 24, and separators 26 and 27. The anode 23 and thecathode 24 are diffusion electrodes making up a sandwich structure inwhich the electrolytic membrane 22 is sandwiched between the anode 23and the cathode 24, which are arranged opposite each other. Theseparators 26 and 27, composed of a non-air-permeable conductivematerial, further sandwich the sandwich structure therebetween so as toform channels for the fuel gas between the separator 26 and the anode 23and channels for the oxidation gas between the separator 27 and thecathode 24. Ribs 26 a with a recess cross section are formed in theseparator 26. The anode 23 abuts against the ribs 26 a to close theopenings of the ribs 26 a to form fuel gas channels. Ribs 27 a with arecess cross section are formed in the separator 27. The cathode 24abuts against the ribs 27 a to close the openings of the ribs 27 a toform oxidation gas channels.

The anode 23 is composed mainly of carbon powder carrying aplatinum-containing metal catalyst. The anode 23 includes a catalystlayer 23 a that is in contact with the electrolytic membrane 22 and agas diffusion layer 23 b formed on a surface of the catalyst layer 23 aand which is both air permeable and electronically conductive.Similarly, the cathode 24 includes a catalyst layer 24 a and a gasdiffusion layer 24 b. More specifically, the catalyst layers 23 a and 24a are formed by dispersing, in an appropriate organic solvent, carbonpowder carrying platinum or an alloy made up of platinum and anothermetal, adding an appropriate amount of electrolytic solution to thesolvent to obtain a mixture like paste, and applying the mixture ontothe electrolytic membrane 22. The gas diffusion layers 23 b and 24 b areformed of carbon cloth, carbon paper, or carbon felt which is obtainedby weaving yarns made up of carbon fibers. The electrolytic membrane 22is a proton conductive ion exchange membrane formed of a solid polymermaterial, for example, a fluorine-containing resin. The electrolyticmembrane 22 exhibits high electric conductivity in a wet condition. Theelectrolytic membrane 22, the anode 23, and the cathode 24 form amembrane-electrode assembly 25.

EMBODIMENT 1

Now, cell voltage management according to Embodiment 1 will be outlined.

The controller 70 performs operation control while limiting theoperation so as to prevent the cell voltage from decreasing below alower voltage limit Vth. The controller 70 changes conditions for theoperation limitation depending on the operation condition of the fuelcell stack 20. The operation condition is a concept collectivelyreferring to physical quantities relating to the cell voltage reductionfactors (a shortage of the supplied reaction gas, a decrease in stacktemperature, and the like). The conditions for the operation limitationrefer to conditions for limiting the cell operation (for example, thelower voltage limit Vth, indicating the lower limit of the powergeneration tolerable range). The severity of the conditions for theoperation limitation is desirably reduced consistently with the stacktemperature. For example, the lower voltage limit Vth forlow-temperature starting is desirably set to be lower than that for thenormal operation (the normal operation refers to a cell operationperformed within a temperature range suitable for the cell operationafter the warming-up operation is completed. The normal operation issynonymous with normal power generation. The low-temperature startingrefers to starting requiring the warming-up operation because thetemperature during the starting is equal to or lower than apredetermined temperature). The lower voltage limit Vth may be providedwith temperature characteristics so as to decrease gradually withtemperature decrease.

If the cell voltage reduction factor is the shortage of the fuel gassupplied to the anode 23, continuous power generation with the cell 21offering a reverse potential may severely damage the cell 21. Thus, thelower voltage limit Vth is set to be higher to strictly limit a decreasein cell voltage so as to prevent the possible reverse potential. Thelower voltage limit Vth is desirably set to a value increasingconsistently with the shortage amount of the supplied hydrogen withrespect to the amount of hydrogen required for adequate reaction withoxygen (that is, a theoretical value).

On the other hand, if the cell voltage reduction factor is the shortageof the oxidation gas supplied to the cathode 24, the continuous powergeneration with the cell 21 offering the reverse potential less severelydamages the cell 21 than when the fuel gas is in shortage. Thus, toallow the reverse potential to be generated, the lower voltage limit Vthfor the oxidation gas shortage is preferably set to be lower than thatfor the fuel gas shortage to relatively ease the limitation on thedecrease in cell voltage. When the amount of the supplied oxygen is muchbelow the amount of oxygen required for adequate reaction with hydrogen(that is, the theoretical value), the lower voltage limit Vth isdesirably set to be higher than that set when the shortage is small.

A relationship between the cell voltage reduction factor and the lowervoltage limit Vth will be summarized. Under the condition that thetemperature observed when the fuel gas is in shortage is the same asthat observed when the oxidation gas is in shortage, the fuel gasshortage may more severely damage the fuel cell stack 20 than theoxidation gas shortage. Thus, the lower voltage limit Vth for the fuelgas shortage is desirably set to be higher than that for the oxidationgas shortage.

FIG. 3 is a flowchart showing a cell voltage management routine for thenormal operation.

The routine is invoked at equal period intervals during the normaloperation and repeatedly executed by the controller 70. The routinetreats the lower voltage limit Vth as a variable, Vth0 as a constantdetermined by the temperature (the lower voltage limit provided when thereaction gas is sufficiently supplied), Vh as a constant (the amount bywhich the lower voltage limit is increased) determined according to theshortage amount of the supplied hydrogen with respect to the amount ofhydrogen required for adequate reaction with oxygen (that is, thetheoretical value), and Va as a constant (the amount by which the lowervoltage limit is increased) determined according to the shortage amountof the supplied oxygen with respect to the amount of oxygen required foradequate reaction with hydrogen (that is, a theoretical value). Theroutine updates the value of the lower voltage limit Vth at everyarithmetic period.

The controller 70 determines whether or not the fuel gas is in shortage(step 301). If the fuel gas is in shortage (step 301; YES), thecontroller 70 updates the value of the lower voltage limit Vth by addingVh to Vth0 (step 305). At this time, the value Vh is increasedconsistently with the shortage amount of the supplied hydrogen withrespect to the amount of hydrogen required for adequate reaction withoxygen (that is, the theoretical value). As a preferred embodiment ofthe present invention, proportional control is preferable whichincreases the value Vh in proportion to the shortage amount of thesupplied hydrogen.

In this case, the value Vh is set to be larger than the value Va underthe condition that the temperature observed when the fuel gas is inshortage is the same as that observed when the oxidation gas is inshortage.

If the fuel gas is not in shortage (step 301; NO), the controller 70determines whether or not the oxidation gas is in shortage (step 302).If the oxidation gas is in shortage (step 302; YES), the controller 70updates the value of the lower voltage limit Vth by adding Va to Vth0(step 304). At this time, the value Va is increased consistently withthe shortage amount of the supplied oxygen with respect to the amount ofoxygen required for adequate reaction with hydrogen (that is, thetheoretical value). As a preferred embodiment of the present invention,proportional control is preferable which increases the value Va inproportion to the shortage amount of the supplied oxygen.

If the oxidation gas is not in shortage (step 302; NO), the controller70 updates the value of the lower voltage limit Vth to Vth0 (step 303).

The controller 70 then determines whether or not the cell voltage islower than the lower voltage limit Vth (step 306). If the cell voltageis equal to or higher than the lower voltage limit Vth (step 306; NO),the controller 70 exits the cell voltage management routine. If the cellvoltage is lower than the lower voltage limit Vth (step 306; YES), thecontroller 70 executes a cell voltage recovery process (step 307).

Preferably, the cell voltage recovery process is one of a process forcompensating for the reaction gas shortage (to compensate for the fuelgas shortage, for example, a process is executed which controls theinjector 44 so as to increase the supply pressure of the fuel gassupplied to the fuel cell stack 20 or which controls the rotation speedof the circulation pump 47 so as to increase the flow rate of the fuelgas flowing into the fuel cell stack 20. To compensate for the oxidationgas shortage, a process is executed which, for example, controls therotation speed of the air compressor 32 so as to increase the supplyamount of the oxidation gas flowing into the fuel cell stack 20.), aprocess of limiting the output current from the fuel cell stack 20 (forexample, a process of controlling the DC/DC converter 51 so as to limita current obtained from the fuel cell stack 20); a process of stoppingthe power generation. One of these processes is executed to attempt torecover the cell voltage.

Since the output from the fuel cell stack 20 is limited, the output fromthe fuel cell stack 20 may not meet a power requirement for the system.In this case, the shortage of power is compensated for by the battery52.

Now, a process of determining whether or not the reaction gas is inshortage will be described with reference to FIGS. 4 and 5. Thedetermination process is executed in steps 301 and 302, described above.

FIG. 4 shows an I-V characteristic (current-voltage characteristic ofthe fuel cell stack 20 observed when the reaction gas is in shortage ina transitive power generation condition with a variation load. A solidline indicates the characteristic observed when the fuel gas is inshortage. A dashed line indicates the I-V characteristic observed whenthe oxidation gas is in shortage. As seen in this graph, |ΔV/ΔI|observed when the fuel gas is in shortage is larger than that observedwhen the oxidation gas is in shortage. The reason can be explained asfollows.

When the fuel gas is in shortage, a power generation current cannot bemaintained only by protons resulting from the oxidation reaction(H₂→2H⁺+2e⁻) in the anode 23. Thus, the cell 21 generates protonsthrough electrolysis of water in order to compensate for the fuel gasshortage. At this time, the cell 21 electrically decomposes the water byobtaining energy from another cell 21 to rapidly increase the anodepotential up to a potential at which the electrolysis of waterprogresses. As a result, the anode potential increases rapidly above thecathode potential to rapidly make the cell voltage negative (the cellvoltage exhibits the reverse potential). This means that |ΔV/ΔI|observed when the fuel gas is in shortage is larger than that observedwhen the fuel gas is not in shortage. It should be noted that if Vdecreases, ΔV/ΔI has a negative value.

A change in voltage is preferably detected near 0 V to determine whetherthe reverse potential results from the fuel gas shortage or theoxidation gas shortage. For example, detecting a change in voltage at0.6 V or lower enables determination of whether the reverse potential,corresponding to a voltage change of a large absolute value, resultsfrom the fuel gas shortage or the oxidation gas shortage.

When the anode potential is high, the platinum catalyst may be ionizedand eluted or the carbon carrying the platinum catalyst may be oxidized.These phenomena may degrade the performance of the catalyst. Thus, thepossible reverse potential needs to be avoided.

On the other hand, when the oxidation gas is in shortage, protonspassing from the anode 23 through the electrolytic membrane 22 to thecathode 24 cannot react with oxygen. Thus, the protons bond to electronsflowing through an external circuit (power line 50) to generate hydrogen(2H⁺+2e⁻→H²). In this case, since the ohmic resistance of theelectrolytic membrane 22 is dominant, there is no significant differencebetween |ΔV/ΔI| observed when the oxidation gas is in shortage and|ΔV/ΔI| observed when the oxidation gas is not in shortage.

The above discussions will be summarized. In the transitive powergeneration condition with a variation in load, Formula (4) holds truewhen the fuel gas is in shortage, and Formula (5) holds true when theoxidation gas is in shortage. Here, a threshold X(T) is a function of atemperature T.

|ΔV/ΔI|≧X(T)   (4)

|ΔV/ΔI|<X(T)   (5)

FIG. 5 shows a V-t characteristic (voltage-time characteristic) of thefuel cell stack 20 observed when the reaction gas is in shortage in thesteady-state power generation condition with no variation in load. Evenwith a constant load (load current), the gas containing moisture flowsconstantly through the gas channels inside the fuel cell stack 20. Thus,for example, large droplets may cover a surface of the electrode tocause accidental flooding, resulting in a temporary reaction gasshortage. A solid line indicates the V-t characteristic observed whenthe fuel gas is in shortage. A dashed line indicates the V-tcharacteristic observed when the oxidation gas is in shortage. As seenin this graph, |ΔV/Δt| observed when the fuel gas is in shortage islarger than that observed when the oxidation gas is in shortage. Thereason can be explained as is the case with the variation in the I-Vcharacteristic of the fuel cell stack 20 observed when the reaction gasis in shortage in the transitive power generation condition with avariation in load. It should be noted that if V decreases, ΔV/Δt has anegative value.

The above discussions will be summarized. In the steady-state powergeneration condition with no variation in load, Formula (6) holds truewhen the fuel gas is in shortage, and Formula (7) holds true when theoxidation gas is in shortage. Here, a threshold Y(T) is a function ofthe temperature T.

|ΔV/Δt≧Y(T)   (6)

|ΔV/Δt|<Y(T)   (7)

As is appreciated from the above description, the controller 70functions as a determination device that determines whether the cellvoltage reduction factor is the fuel gas shortage or the oxidation gasshortage, a lower voltage limit setting device that sets the lowervoltage limit according to the cell voltage reduction factor, and acontrol device that controls the output voltage from the fuel cell stack20 so as to prevent the cell voltage from decreasing below the lowervoltage limit.

EMBODIMENT 2

Cell voltage management in a low-temperature environment according toEmbodiment 2 will be described.

FIG. 6 is a flowchart showing a low-temperature starting routine. Theroutine is invoked during a low-efficiency operation and executed by thecontroller 70. The low-efficiency operation refers to an operation ofcontrolling the supply amount of the reaction gas supplied to the fuelcell stack 20 with an air stoichiometry set to about 1.0 so as toincrease a power loss for operation at a low power generationefficiency. The air stoichiometry refers to an oxygen excess rate, andindicates how excess the supplied oxygen is with respect to the amountof oxygen required for adequate reaction with hydrogen. Performing thelow-efficiency operation with the air stoichiometry set to a small valueincreases concentration overvoltage and thus a heat loss (power loss)contained in energy obtained through the reaction between hydrogen andoxygen. The low-efficiency operation is utilized during, for example,low-temperature starting to intentionally increase the heat loss toquickly warm up the fuel cell stack 20.

Upon receiving a starting signal IG output by an ignition switch (step601; YES), the controller 70 reads a detection value from a temperaturesensor 74 to determine whether or not a refrigerant temperature T islower than a threshold temperature T0 (step 602). The thresholdtemperature TO is a temperature based on which whether or not to performthe low-efficiency operation is determined. The threshold temperature T0is set to, for example, about 0° C.

If the refrigerant temperature T is not lower than the thresholdtemperature T0 (step 602; NO), the low-efficiency operation need not beperformed. Thus, the controller 70 exits the low-temperature startingroutine, and executes a normal starting process routine (not shown inthe drawings).

On the other hand, if the refrigerant temperature T is lower than thethreshold temperature T0 (step 602; YES), the controller 70 performs thelow-efficiency operation with the valve opening degree of the throttlevalve 35 reduced to set the air stoichiometry to about 1.0 (step 603).The fuel cell stack 20 is then warmed up by the heat loss (heat energy)resulting from the low-efficiency operation.

The controller 70 updates the lower voltage limit Vth by subtracting Vafrom Vth0 (step 604). At this time, the value Va is increased withdecreasing temperature.

The controller 70 then determines whether or not the cell voltage islower than the lower voltage limit Vth (step 605). If the cell voltageis lower than the lower voltage limit Vth (step 605; YES), thecontroller 70 executes the cell voltage recovery process (step 606). Ifthe cell voltage is equal to or higher than the lower voltage limit Vth(step 605; NO), the controller 70 skips the cell voltage recoveryprocess.

The controller 70 reads the detection value from the temperature sensor74 to determine whether or not the refrigerant temperature T is higherthan a threshold temperature T1 (step 607). The threshold temperature T1is a temperature based on which whether or not the warm-up operation iscompleted is determined.

If the refrigerant temperature T is not higher than the thresholdtemperature T1 (step 607; YES), the controller 70 returns to theprocessing in step 603. If the refrigerant temperature T is higher thanthe threshold temperature T1 (step 607; NO), the controller 70 exits thelow-temperature starting routine.

During the low-efficiency operation of intentionally causing theoxidation gas shortage in the low-temperature environment as describedabove, possible damage on the cell 21 by continuous power generationwith the cell 21 offering the reverse potential is less severe than thaton the cell 21 by the oxidation gas shortage during the normaloperation. Thus, to allow the reverse potential to be generated, thelower voltage limit Vth set for the low-efficiency operation is adjustedto be lower than that set for the normal operation to ease thelimitation on the decrease in cell voltage.

In the description of Embodiment 1, the lower voltage limit Vth set forthe fuel gas shortage is adjusted to be higher than the lower voltagelimit Vth set for the oxidation gas shortage (steps 304 and 305). Thisis based on the fact that the temperature observed when the fuel gas isin shortage is the same as that observed when the oxidation gas is inshortage. Under the condition that the temperature is constant, thepossible damage on the fuel cell stack 20 by the fuel gas shortage isseverer than that on the fuel cell stack 20 by the oxidation gasshortage. Thus, the lower voltage limit Vth needs to be adjustedaccording to the cell voltage reduction factor. In contrast, inEmbodiment 2, the lower voltage limit Vth set for the low-efficiencyoperation is adjusted to be lower than that set for the normal operation(step 604). The reason is that during the low-efficiency operation, thetemperature of the fuel cell stack 20 is lower than that during thenormal operation, reducing the activity of the electrochemical reaction,which may damage the fuel cell stack 20.

Thus, additionally, under the condition that the temperature observedwhen the fuel gas is in shortage is the same as that observed when theoxidation gas is in shortage, adjusting the lower voltage limit Vth setfor the low-efficiency operation to be lower than that set for thenormal operation (step 604) is consistent with adjusting the lowervoltage limit Vth set for the fuel gas shortage to be higher than thatset for the oxidation gas shortage (steps 304 and 305).

In Embodiments 1 and 2, whether or not the cell voltage is lower thanthe lower voltage limit Vth may be determined (steps 306 and 606) forthe cell voltage of each cell 21 or for the sum of the cell voltages ofa plurality of the cells 21.

With the fuel cell system 10 according to Embodiments 1 and 2, the lowervoltage limit is changed depending on the cell voltage reduction factor.Consequently, the tolerable range of power generation can be flexiblychanged.

EMBODIMENT 3

Now, cell voltage management according to Embodiment 3 will bedescribed. The cell voltage management described below is assumed to beused mainly for the normal operation. However, owing to the lack of thetemperature limitation, the cell voltage management can in principle beapplied to the low temperature environment (for example, during thelow-efficiency operation).

(1) Cell Voltage Management Based on the Cell Electrode Potential

FIG. 7 shows a variation in electrode potential, that is, the electrodepotentials measured during the starting and the normal operation andwhen the oxidation gas is in shortage and when the fuel gas is inshortage. During the starting, the output voltage from the fuel cellstack 20 is set to an open end voltage, resulting in a zero outputcurrent. At this time, a cathode potential CA1 exhibits no voltage dropcaused by a DC resistance component and is set to the open end voltage.Furthermore, an anode potential AN1 remains zero. A cell voltage VC1during the starting is equal to a voltage value (open end voltage)obtained by subtracting the anode potential AN1 during the starting fromthe cathode potential CA1 during the starting. The cell voltage VC1 perunit cell is about 1.0 V.

During the normal operation, the anode potential AN2 increases up to apredetermined positive value determined by the electrochemical reaction.On the other hand, a cathode potential CA2 exhibits a voltage drop IR2caused by the DC resistance component. A cell voltage VC2 during thenormal operation is equal to a voltage value VC2 obtained by subtractingthe anode potential AN2 during the normal operation from the cathodepotential CA2 during the normal operation. The cell voltage VC2 per unitcell is about 0.6 V.

When the oxidation gas is in shortage, comparison of an anode potentialAN3 with the anode potential AN2 during the normal operation indicatesalmost no potential variation. On the other hand, a cathode potentialCA3 is lower than the cathode potential CA2 during the normal operationand exhibits a voltage drop IR3 caused by the DC resistance component.Thus, the cathode potential CA3 is lower than the anode potential AN3. Acell voltage VC3 measured when the oxidation gas is in shortage is equalto a voltage value VC3 obtained by subtracting the anode potential AN3measured when the oxidation gas is in shortage from the cathodepotential CA3 measured when the oxidation gas is in shortage. Since thecathode potential CA3 is lower than the anode potential AN3, the cellvoltage VC3 has a negative value.

When the fuel gas is in shortage, a cathode potential CA4 exhibits avoltage drop IR4 caused by the DC resistance component. However,comparison of the cathode potential CA4 with the cathode potential CA2during the normal operation indicates almost no potential variation. Onthe other hand, an anode potential AN4 is higher than the anodepotential AN2 during the normal operation. Thus, protons are generatedthrough the electrolysis of water to compensate for the fuel gasshortage. A cell voltage VC4 measured when the fuel gas is in shortageis equal to a voltage value VC4 obtained by subtracting the anodepotential AN4 measured when the fuel gas is in shortage from the cathodepotential CA4 measured when the fuel gas is in shortage. Since thecathode potential CA4 is lower than the anode potential AN4, the cellvoltage VC4 has a negative value.

Here, it should be noted that when the oxidation gas is in shortage, theanode potential AN3 is not higher than a predetermined thresholdpotential Van but that when the fuel gas is in shortage, the anodepotential AN4 is higher than the threshold potential Van because protonsare generated by the electrolysis of water. The threshold potential Vanmay be set to a value within a potential range reached by the anodepotential during the electrolysis of water, the value being close to apotential (for example, 1.3 V) at which the platinum catalyst containedin the catalyst layer of the membrane-electrode assembly 25 starts to beionized. By providing the fuel cell stack 20 with a measurement circuit(not shown in the drawings) that measures the anode potential, thecontroller 70 can determine whether or not the reaction gas is inshortage based on the anode potential.

For example, the controller 70 monitors an output signal from the cellmonitor 73 at predetermined arithmetic period intervals. Upon detectingthat the cell voltage has decreased down to a negative value, thecontroller 70 compares the anode potential with the threshold potentialVan. When the comparison indicates that the anode potential is higherthan the threshold potential Van, the controller 70 determines that thefuel gas is in shortage. When the comparison indicates that the anodepotential is lower than the threshold potential Van, the controller 70determines that the oxidation gas is in shortage. This determinationprocess is applicable to the fuel gas shortage determination process(step 301) in Embodiment 1 and the oxidation gas shortage determinationprocess (step 302). When the controller 70 determines that the fuel gasis in shortage, the processing in steps 305, 306, and 307 in Embodiment1 is executed. When the controller 70 determines that the oxidation gasis in shortage, the processing in steps 304, 306, and 307 in Embodiment1 is executed. However, when the oxidation gas is in shortage, the cell21 is prevented from being electrochemically damaged. The cell voltagerecovery process (step 307) such as the output limitation process may beomitted to allow the cell voltage to exhibit a negative value.

The above-described cell voltage management based on the cell electrodepotential enables determination of whether or not the cell voltagereduction factor is the fuel gas shortage or the oxidation gas shortage.Thus, when the controller determines that oxidation gas is in shortage,the output limitation may be avoided to allow the cell voltage todecrease. Therefore, possible degradation of drivability can be avoided.

(2) Cell Voltage Management Based on the I-V Characteristic Map

FIG. 8 shows an I-V characteristic map of the cell 21. The axis ofabscissa indicates the cell current, and the axis of ordinate indicatesthe cell voltage. An I-V characteristic map 801 indicates the I-Vcharacteristic observed during the normal operation on the assumptionthat an appropriate amount of reaction gas is supplied to the cell 21.An I-V characteristic map 802 indicates the I-V characteristic observedon the assumption that an insufficient amount of fuel gas is supplied tothe cell 21 and that a sufficient amount of moisture required togenerate protons so as to compensate for the fuel gas shortage is storedinside the fuel cell stack 20. In such a situation, the sufficientamount of moisture required to generate protons is present, thusminimizing the possible decrease in cell voltage during the electrolysisof water. Here, it should be noted that even if the cell voltage dropsto a negative value owing to any factor, the cell voltage reductionfactor is ensured not at least to be the fuel gas shortage as long asthe current cell voltage belongs to a tolerable range 803. Here, thetolerable range 803 is such that within the range, the cell voltageexhibits a negative value, and corresponds to an upper part of the I-Vcharacteristic map 802. However, it should be noted that even if thecell voltage decreases below the I-V characteristic map 802, this doesnot necessarily indicate that the fuel gas shortage is occurring. Thischaracteristic can be utilized to determine whether or not the fuel gasis in shortage.

For example, the controller 70 monitors the output signal from the cellmonitor 73 at predetermined arithmetic period intervals. Upon detectingthat the cell voltage has dropped to a negative value, the controller 70compares the voltage value of the I-V characteristic map 802 (the cellvoltage on the I-V characteristic map 802 corresponding to the cellcurrent measured when the cell voltage drops to the negative voltage)with the actual cell voltage. If the comparison indicates that theactual cell voltage is higher than the voltage value of the I-Vcharacteristic map 802 (if the actual cell voltage belongs to thetolerable range 803), the controller 70 determines that the fuel gas isnot in shortage. This determination process is applicable to the fuelgas shortage determination process (step 301) in Embodiment 1. When thecontroller 70 determines that the fuel gas is in shortage, theprocessing in steps 305, 306, and 307 in Embodiment 1 is executed. Whenthe fuel gas is not in shortage, the decrease in cell voltage isprevented from damaging the cell 21 and is thus allowed. Moreover, theneed for the output limitation on the fuel cell stack 20 is eliminated.The I-V characteristic map 802 may be held in a memory accessible to thecontroller 70.

Now, variations of the cell voltage management based on the I-Vcharacteristic will be described with reference to FIGS. 9 to 11.

FIG. 9 shows the I-V characteristic map 802 for which temperaturecharacteristics are taken into account. I-V characteristic maps 802A,802B, and 802C indicate I-V characteristics under different temperatureenvironments observed on the assumption that an insufficient amount offuel gas is supplied to the cell 21 and that a sufficient amount ofmoisture required to generate protons so as to compensate for the fuelgas shortage is stored inside the fuel cell stack 20. The absolute valueof an activation overvoltage increases with decreasing temperature. Thisincreases the degree of the decrease in cell voltage in a low currentregion. Furthermore, proton transportation resistance increases withdecreasing temperature. This increases the inclination of a linearregion of the I-V characteristic. This is why the I-V characteristic map802B indicates an I-V characteristic observed in an environment colderthan that for the I-V characteristic map 802A and the I-V characteristicmap 802C indicates an I-V characteristic observed in an environmentcolder than that for the I-V characteristic map 802B. As shown in theI-V characteristic maps 802A, 802B, and 802C, the tolerable range 803 ofthe decrease in cell voltage increases gradually with decreasingtemperature. Using the I-V characteristic map 802, for which thetemperature characteristics are taken into account, enables closer cellvoltage management.

FIG. 10 shows an I-V characteristic map 804 obtained through correctionstaking into account a margin 805 obtained by converting control delaytime into a voltage value. When electrolysis is applied to water whilethe fuel gas is in shortage, the cell voltage decreases at a relativelyrapid rate. Thus, even when the controller determines that the fuel gasis not in shortage after the cell voltage has dropped to a value withinthe tolerable range 803 and relatively close to the I-V characteristicmap 802, if the fuel gas is actually in shortage, the cell voltage maythereafter decrease significantly below the I-V characteristic map 802in a short time. The sharp reduction in cell voltage may increase theanode potential above the above-described threshold potential Van toionize the platinum catalyst, contained in the catalyst layer of themembrane-electrode assembly 25. The possible control delay can beavoided by correcting the I-V characteristic map 802 taking into accountthe margin 805, obtained by converting the control delay time into thevoltage value, and using the I-V characteristic map 804, obtainedthrough the corrections, to determine whether or not the reaction gas isin shortage. The control delay time may be the duration of the variousprocess (cell voltage sampling time, reaction gas shortage determinationtime, and the like) required for the cell voltage management, amechanical delay time of responses from a control system, or the like

FIG. 11 shows a simplified I-V characteristic map 806. The I-Vcharacteristic map 806 is map data exhibiting a constant value V0regardless of the cell current. The constant value V0 can be determinedto be a cell voltage corresponding to a cell current=0 on the I-Vcharacteristic map 802. By setting the highest voltage value V0 on theI-V characteristic map 802 to be a threshold voltage based on whichwhether or not the fuel gas is in shortage, whether or not the fuel gasis in shortage can be determined for the entire cell current range basedon the voltage V0. Thus, the cell voltage management can be simplified.In this case, the voltage value V0 corresponding to each of thedifferent temperatures may be pre-calculated so that the cell voltagecan be managed based on the I-V characteristic map 806, for which thetemperature characteristics are taken into account. Alternatively,voltage values based on which whether or not the fuel gas is in shortagemay be pre-calculated for the entire temperature range assumed for thecell operation so that the voltage values obtained constitute the I-Vcharacteristic map 806.

The process of determining whether or not the fuel gas is in shortageusing following is the same as the process of determining whether or notthe fuel gas is in shortage using the I-V characteristic map 802 (FIG.8): the I-V characteristic maps 802A, 802B, and 802C (FIG. 9), for whichthe temperature characteristics are taken into account, the I-Vcharacteristic map 804 (FIG. 10), for which the control time delay istaken account, or the simplified I-V characteristic map 806 (FIG. 11).The I-V characteristic maps 802 (802A, 802B, and 802C), 804, and 806,shown in FIGS. 9 to 11, need not necessarily be used independently butmay be used with any of the features of the I-V characteristic mapscombined together.

According to the above-described cell voltage management based on theI-V characteristic map, the tolerable range 803 of the cell voltagewithin which the fuel gas is ensured not to be in shortage can bedetermined using the I-V characteristic maps 802, 804, and 806 withwhich the decrease in cell voltage is minimized during the electrolysisof water. When the fuel gas is determined not to be in shortage, thedecrease in cell voltage can be allowed, eliminating the need for theoutput limitation. Therefore, possible degradation of drivability can beavoided.

In Embodiments 1 to 3, described above, the configuration in which thefuel cell system 10 is used as a vehicle-mounted power supply system isillustrated. However, the configuration of the fuel cell system 10 isnot limited to this example. For example, the fuel cell system 10 may bemounted as a power source for any mobile object (a robot, a ship, anairplane, or the like) other than the fuel cell vehicle. Alternatively,the fuel cell system 10 may be used as a power generation facility(stationary power generation system) for a house, a building, or thelike.

INDUSTRIAL APPLICABILITY

According to the present invention, the tolerable range of powergeneration can be flexibly changed by changing the lower voltage limitdepending on the operation condition of the fuel cell. Furthermore,according to the present invention, the cell voltage can be properlymanaged according to the cell voltage reduction factor by determiningwhether the cell voltage reduction factor is the fuel gas shortage orthe oxidation gas shortage.

1. A fuel cell system comprising: a fuel cell which receives a supply ofa fuel gas and an oxidation gas to generate power; a determinationdevice which determines whether or not an operation condition of thefuel cell corresponds to a fuel gas shortage or an oxidation gasshortage; a lower voltage limit setting device which, when thedetermination device determines that the fuel gas is in shortage, sets alower voltage limit to be higher than that set when the determinationdevice determines that the oxidation gas is in shortage; and a controldevice which controls an output voltage from the fuel cell so as toprevent the output voltage from decreasing below the lower voltage limitset by the lower voltage limit setting device.
 2. A fuel cell systemcomprising: a fuel cell which receives a supply of a fuel gas and anoxidation gas to generate power; a lower voltage limit setting devicewhich sets a lower voltage limit for a low-efficiency operation to belower than that for a normal operation; and a control device thatcontrols an output voltage from the fuel cell so as to prevent theoutput voltage from decreasing below the lower voltage limit set by thelower voltage limit setting device.
 3. The fuel cell system according toclaim 2, wherein the control device performs the low-efficiencyoperation when temperature is equal to or lower than a predeterminedvalue at the time of the starting of the fuel cell system.
 4. The fuelcell system according to claim 2, wherein the lower voltage limitsetting device sets the lower voltage limit to be lower when the fuelgas is in shortage during the low-efficiency operation than when thefuel gas is in shortage during the normal operation.
 5. The fuel cellsystem according to claim 1, wherein the lower voltage limit settingdevice sets the lower voltage limit to have a lower value as thetemperature of the fuel cell decreases.
 6. The fuel cell systemaccording to claim 1, wherein when an output voltage from the fuel cellis lower than the lower voltage limit, the control device carries outone of a process of an increasing amount of fuel gas or oxidation gassupplied to the fuel cell, a process of limiting the output current fromthe fuel cell, and a process of stopping power generation.
 7. The fuelcell system according to claim 1, wherein in a transitive powergeneration condition with a variation in load, when a variation in theoutput voltage from the fuel cell is defined as ΔV and a variation inoutput current from the fuel cell is defined as ΔI, the determinationdevice calculates |ΔV/ΔI|, and when the |ΔV/ΔI| is equal to or largerthan a first threshold, determines that the fuel gas is in shortage, andwhen the |ΔV/ΔI| is smaller than the first threshold, the determinationdevice determines that the oxidation gas is in shortage.
 8. The fuelcell system according to claim 1, wherein in a steady-state powergeneration condition with no variation in load, when a variation in theoutput voltage from the fuel cell is defined as ΔV and a temporalvariation is defined as Δt, the determination device calculates |ΔV/Δt|,and when the |ΔV/Δt| is equal to or larger than a second threshold,determines that the fuel gas is in shortage, and when the |ΔV/Δt| issmaller than the second threshold, the determination device determinesthat the oxidation gas is in shortage.
 9. The fuel cell system accordingto claim 1, wherein when an anode potential of the fuel cell is higherthan a predetermined threshold voltage, the determination devicedetermines that the fuel gas is in shortage, and when the anodepotential of the fuel cell is lower than the predetermined thresholdvoltage, the determination device determines that the oxidation gas isin shortage.
 10. The fuel cell system according to claim 1, wherein thedetermination device calculates a cell voltage on a current-voltagecharacteristic map created under a condition which minimizes a decreasein the anode potential of the fuel cell during the electrolysis ofwater, and when the actual cell voltage is higher than the cell voltageon the current-voltage characteristic map, the determination devicedetermines that the fuel gas shortage is not occurring.
 11. The fuelcell system according to claim 10, wherein the current-voltagecharacteristic map is map data with temperature characteristics.
 12. Thefuel cell system according to claim 10, wherein the current-voltagecharacteristic map is map data pre-corrected taking into account controldelay time required to control the cell voltage.
 13. The fuel cellsystem according to claim 10, wherein the current-voltage characteristicmap is map data exhibiting a constant voltage value regardless of cellcurrent.